Bit assemblies have long been utilized in road and highway milling machinery, as well as in off-road trenching equipment and in mining machinery. On such machinery, a plurality of bit assemblies are mounted both across the width and around the perimeter, sometimes in spiral or herringbone orientation, on the outside of a hollow rotary drum. Such bit assemblies are also utilized on the outside of a continuous chain, or similar endless looping machinery where the bits are moved through an orbit that is intercepted by the face of the road material being milled, the earth material from which a trench is being dug, and the material being mined.
The bit assemblies include a bit that has a working end and a shank. The shank is received in and may also be rotatably mounted in a bit holder that is secured, in turn, onto a bit block mounted (usually welded) on the outside of the drum or welded on a stand which is welded to a drum. Bits typically have a hardened working end, preferably made of tungsten carbide or other hardened material, which impinges and digs into the surface it contacts to remove a portion of same. By utilizing a plurality of the bit assemblies around the outer surface of such a rotating drum or continuous chain, the amount of material removal in a given period of time may be substantial.
While bit assemblies include a bit and a bit block, they may include an intermediate member, depending on the intended application and the severity of work to be expended. For light duty applications and especially for down drilling applications such as foundation work where the forces on the bit are mostly axial, sheet metal fashioned retainers may be utilized. For heavy duty applications such as road milling and mining operations where the bit receives forces from multiple angles during operation, a forged annular bit holder is used.
A retainer is usually made of sheet spring steel having at least 0.5% carbon content (a differing material than the bit and bit block), a maximum thickness of about 0.050 inch, and is rolled in a circular discontinuous shape to elastically fit around the shank of the bit and within the bore of a bit block. The engineering concepts used to create retainers and bit holders are quite different. The retainer, and the bit, can rotate in the bit block. Usually, the bit shank is received in the bit block to allow space for the retainer to be mounted therearound. Retainer applications are limited to lighter duty applications or high axial loading applications. A force of about 70 pounds can remove a retainer and bit assembly from around a bit block. Thin walled retainers are not as long-lived as bit holders because lime and caustic action in the concrete or asphalt aggregate eats away the steel.
A bit holder is typically made of forged steel, although it could be machined from bar stock or formed of powdered metal at more expense. The holder can be made of the same material as the bit block, typically 4140, 8640, 4340 or similar steels having less than 0.5% carbon content. A bit holder has an enlarged forward end that extends beyond the top of a bit block, positioning a bit substantially more forward than a retainer and adding bulk material for a stronger single member part capable of absorbing substantial forces from many angles. A bit holder does not rotate in the bit holder block.
Since such road milling, trenching and mining machinery is considered heavy duty earth, coal, mineral or macadam removal machinery, substantial forces will operate on the bit assemblies in question. Engineers and operators of such equipment have long sought to extend the working life of these components and decrease the down time of such equipment. A major breakthrough in the longevity of use of such equipment and in decreasing the time necessary to replace worn or broken bit assemblies used on such equipment was made by utilizing the bit assemblies shown and disclosed in U.S. Pat. No. 6,585,326 issued Jul. 1, 2003. That patent disclosed a bit assembly utilizing a bit holder that was held in place in its bit block without the necessity of utilizing a nut, retaining clip, bolt, or the like to maintain the bit holder in operative position in its bit block.
By providing a bit holder with a generally cylindrical hollow shank having an elongate slot axially positioned through one side of the shank from the distal end thereof and extending toward the forward body portion of the shank, the bit holder was able to be pressed or driven into a bore of the bit holder block such that the outer, generally cylindrical, radius of the thick walled shank was elastically collapsed an amount that was greater than the interference dimensions of a similarly sized solid shaft. It was found that the bit holder could be maintained in the bit holder block during operation, and be removed and replaced quickly by being driven in or out of its associated bit holder block with a force between 3,000 and 10,000 pounds, without the need of removing retaining clips, threaded nuts or the like.
A standard interference fit as used herein is a heavy duty fit sometimes referred to as force fits or class FN5 force fits. The limits for FN5 force fits are found in numerous engineering handbooks. The quick change bit holder invented by me has utilized a press or force fit greater than a standard interference fit or FN5 force fit, as note in my previous patents.
Additionally, the inventions disclosed in U.S. Pat. No. 7,097,258 issued Aug. 29, 2006 disclose a quick-change bit holder preferably having a slightly tapered shank with a pair of raised outer surfaces on a mediate portion along the length of the shank. The shank also includes a pair of diametrically opposed axially oriented slots extending along the shank through the mediate portion and immediately adjacent the raised outer portions of the shank on either side thereof. In this embodiment, unlike the embodiment first disclosed in U.S. Pat. No. 6,585,326, the dual opposed slots were totally internal in the shank and did not extend to the distal end of the shank.
As such, the distal end of the shank provided more rigidity than the distal end of the shank disclosed in the '326 patent, but allowed enough deformation in the enlarged mediate portion of the shank, when pressed into a bit holder block to maintain the bit holder in a tapered bit holder block bore.
While the preferred embodiments shown in U.S. Pat. Nos. 6,585,326 and 7,097,258, were slightly tapered to the order of 1 degree or less per side, additional disclosures were made not only of tapered shanks, but shanks going from such a taper through and including a concave shape. Such shanks on either side of a strictly cylindrical shank, would be more efficient than a cylindrical shank in a cylindrical bore because the amount of surface contact, i.e., the driving interference distance, of such shanks would be less than the driving interference distance necessary for a completely cylindrical shank. However, such a cylindrical shape shank could work, although less efficiently and with more effort to insert or remove than the other preferred mentioned shanks.
The preferred bit holder blocks shown and utilized in the '326 patent include bores therethrough that are generally cylindrical with preferably a slight taper of 1 degree per side or less (preferably the same taper as the bit holder shank). Bit block bores that are completely cylindrical and also with 3½ degree per side taper have been utilized in bit assemblies. There are solid bit holder shanks and bit holder block assemblies that are press fit assemblies.
A bit holder utilizing a substantial distal shank portion having a straight cylindrical outline and fitting into a bit holder block bore having a straight cylindrical bottom end with a slightly widened top end thereof is shown at U.S. Pat. No. 6,854,810.
Since the bases of all of the known such bit block assemblies are welded to either the outside of a generally cylindrical drum or welded to the outside of links of a chain or similar continuous looping mechanism, and since such bit blocks have substantially more metal material to withstand shock, wear and the like, and since base blocks are shielded by the frontal portion of the holder, bit blocks tend to have a much longer service life than bit holders or bits. As such, the bit holders and bits are made to be replaceable, and the more easily replaceable the better.
As mentioned previously, the use of a quick change type bit holder as disclosed in the '326 patent both lessens down time of its associated machinery, and the additional upper body material of such preferred bit holders lengthens the in-service life thereof.
The use of bit holders with differing bit holder blocks having both slightly tapered bit holder block bores and partially cylindrical bit holder block bores has heretofore meant that when one picked a drum, chain, or the like of one manufacturer, one was limited to that manufacturer's bit holders. Therefore, a need has developed for the construction of a bit holder that may be utilized in either existing type quick change style bit holder blocks.